Cell-permeable. A potent and selective HDAC6 (histone deacetylase 6) inhibitor. Displays 1093-fold selectivity over HDAC1 (IC₅₀ values of 15 nM for HDAC6 vs 16.4 µM for HDAC1). Tubastatin A was substantially more selective than the known HDAC6 inhibitor Tubacin at all isozymes except HDAC8. In addition, it displayed over 1000-fold selectivity against all HDAC isoforms excluding HDAC8, where it displayed 54-fold selectivity. Tubastatin A displayed dose-dependent protection against HCA (homocysteic acid)-induced neuronal cell death starting at 5 µM with near complete protection at 10 µM.